1. Field of the Invention
This invention relates generally to doctor blade holders, and is concerned in particular with an improvement to that class of holders commonly referred to as "finger-type doctor blade holders".
2. The Prior Art
A typical example of a prior art finger-type doctor blade holder is illustrated in FIGS. 1-8. The doctor blade holder includes a finger plate subassembly 10 connected to a tube tray subassembly 12 by means of a full length pivot rod 14. The finger plate subassembly includes a full length loading plate 16 having a plurality of mutually spaced pressure fingers 18 attached to the underside thereof. The loading plate 16 and pressure fingers 18 cooperate in defining noncontinuous recesses 20 adapted to removably receive and support a doctor blade 22. The finger plate subassembly 10 thus constitutes a somewhat flexible structure which displays reactions to applied loads that are governed primarily by the bending and torsional deflection characteristics of the loading plate 16.
The tube tray subassembly 12 includes a full length tray 24 supporting a plurality of mutually spaced upstanding pivot rod brackets 26. Typically, the pressure fingers 18 will be arranged on 2" centers, and the pivot rod brackets will be on 6" centers and located between selected pairs of pressure fingers. Two flexible elongated loading tubes 28 and 30 are loosely retained in the tray on opposite sides of the pivot rod 14. The tray 24 is normally detachably secured to a structural support commonly called a "doctor back" (not shown), and the pivot rod 14 is inserted through aligned holes in the fingers 18 and the brackets 26 to pivotably interconnect the two subassemblies 10, 12.
The loading tubes 28, 30 are connected to a fluid pressure source "S", typically compressed air, with control valves V.sub.1, V.sub.2 interposed therebetween to control the application of pressure to the tubes.
After the blade holder has been mounted on the doctor back and properly aligned with respect to the surface 32 to be doctored, subsequent blade adjustments are made by controlling the differential fluid pressure applied to the loading tubes 28, 30. For example, when a significantly greater fluid pressure is applied to tube 28, the finger plate subassembly 10 is pivoted in a clockwise direction as viewed in FIG. 1 to disengage the doctor blade 22 from the surface 32. This is normally done in order to permit blade edge cleaning or blade replacement. Conversely, when the fluid pressure applied to tube 30 significantly exceeds that applied to tube 28, the finger plate subassembly 10 is pivoted in a counterclockwise direction towards surface 32, thereby producing bending deflections in the doctor blade 22 and loading plate 16 and increasing the loading force that the contacting blade edge applies to surface 32.
Loading tubes used in finger-type doctor blade holders are normally constructed of resilient materials sheathed with a fabric reinforcement which provides flexibility but prevents any significant tube wall elongation. When a tube of this type is internally pressurized and externally unrestrained, it will assume a cylindrical shape as shown in FIG. 4, with a diameter "D". Variations in internal tube pressure do not produce significant variations in this diameter.
However, as shown in FIGS. 3 and 5, if a loading tube such as for example tube 30 is confined between an essentially flat first reaction surface 34 on the underside of a pressure finger 18 and a second parallel reaction surface 36 provided by the bottom of the tube tray 24, and if the distance "H" between the two reaction surfaces is less than the unrestrained diameter D of the inflated tube, the tube will assume a flattened shape which may be described as two opposing semi-cylinders whose centers are separated by a distance "W". The internally applied fluid pressure on the two opposing semi-cylinders are statically balanced and thus do not exert any force on the fingers 18. The distance W is the effective width of the tube where contact exists with the individual fingers. Directly under each finger, the following relationship exists: EQU W=(.pi./2)(D-H)
It thus will be seen that although a loading tube is uniformly pressurized throughout its length, the forces applied to individual fingers will vary roughly inversely to the distance H between each individual finger and the bottom surface of the tube tray.
Between the fingers, as illustrated in FIG. 6, the tube will assume a shape more nearly approaching the normal cylindrical unrestrained shape shown in FIG. 4. The final stabilized tube shapes between the fingers are however, influenced by tensile stresses developed in the reinforcing fabric of the tube sheathe in the direction of the major tube axis, as indicated schematically in FIG. 3 at "T".
The actual cross sectional shape of the tube at any given location between two fingers is not readily predictable due to variables such as the physical characteristics of the tube and its sheathing and the unsupported distance between fingers. Consequently, the shape shown in FIG. 6 is a generalized estimated shape. It is apparent, however, that the effective width W' at locations between the fingers will always be less than the width W under a finger.
Much of the potential loading force of the tube between the fingers is dissipated in producing the longitudinal tension T in the tube and its sheathe. Only a small portion of this tension is resolved into vertically acting forces "V" to provide additional finger loading.
In light of the foregoing, it will be seen that the use of finger-type blade holders does not assure uniformity or consistency of blade loading on the surface 32 to be doctored. For example, as shown in FIG. 7, as the doctor blade 22 wears, the loading tube 30 must expand to maintain blade contact with the surface 32 being doctored. The increase of dimension H results in a reduction of the effective width W, with an accompanying reduction in blade loading.
As shown in FIG. 8, the opposite effect will be experienced if the blade 22 is forced upwardly by the intrusion of foreign material between the surface 32 being doctored and the blade edge. Here, the tube 30 will be flattened to a greater extent as the dimension H decreases, with a resulting increase in effective width W and an increase in blade loading.
Thus it will be seen that should mismatches between the surface 32 and blade profile exist, the inherent flexibility of the finger plate subassembly 10 will usually allow the tube 30 to force a continuous contact by varying the dimension H. Effective tube width W, and consequently blade loading profile, is proportionally varied.
It has now been determined that this relatively high degree of flexibility and inherent ability of the conventional finger-type blade holder to conform may in certain instances be disadvantageous. More particularly, if the doctor blade 22 conforms to an intrusion of foreign material carried under the blade edge by the moving surface 32, the resulting increased load concentration at that point may be insufficient to cut through the foreign material, with the result that the doctoring operation will be seriously impaired. The reason for this is that the tube loading force between the fingers 18 is largely dissipated in creating tensile stresses T in the tube and its sheathing, with only a small portion of the tube loading force being resolved into vertically acting forces V to provide the required additional finger loading.